You can make a stupid adapter that does synchronous calls over the same pipe without any more threads. In JBoss you won't get into trouble if your adapter creates its own threads. The jca 1.5 spec includes explicite thread pooling features (work interface) so that the adapter can use lots of threads but let the app server control their usage.
As a second part of this I have seen in the JCA specification that the application server can share (Optional) the ManagedConnection across multiple connections requests.
Do you have any plans for this to be supported within JBoss?
I have some trouble understanding what the spec is talking about with that. With local transactions, you have to supply handles to the same ManagedConnection for all connection requests within a single tx. Obviously, you can't associate a single connection with more than one tx at a time. I used to encourage xa connections to remain associated with a tx for the shortest possible length of time to improve concurrency, but it turns out that no available xadatasources allow you to do that so I made it optional. (track-connection-by-tx flag).
If you have another scenario that you can describe as connection sharing, please explain. Otherwise, I think we have always shared connections at least as much as possible.